1. Field of the Invention
The present invention relates to a low-profile spread illuminating apparatus used as an illuminating means for signs, various display devices and so on, and used as a back illuminating means and a front illuminating means for a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display (hereinafter referred to as xe2x80x9cLCDxe2x80x9d) operating in low power consumption and characterized by its small thickness and lightweight has been extensively used in electric products including personal computers and cellular phones, and demand for the LCD has been increasing. However, since a liquid crystal which is a structural element of the LCD does not emit light by itself unlike a light emitting element such as a cathode-ray tube (CRT), the LCD requires a separate illuminating means to observe an image when the LCD is used in a dark place. In particular, in recent years, to satisfy a demand for downsizing and energy saving, a spread illuminating apparatus of side light type (light conductive plate type) is often used as an illuminating means for the LCD.
FIG. 9 is a schematic sectional view of a spread illuminating apparatus 1 used as a back illuminating means. The spread illuminating apparatus 1 is mainly composed of a light conductive plate 2 and a lamp 11. A top surface 6 of the light conductive plate 2 is arranged so as to face a bottom surface of a transmission-type liquid crystal display element (not shown). A cold-cathode fluorescent lamp (CCFL) or a hot-cathode fluorescent lamp (HCFL) is used as the lamp 11. Also, recently, a lamp is often used which is configured such that a spot-like light source such as a light emitting diode is arranged at at least one end of a light conductive bar made of a transparent material.
A light reflection pattern 7 is formed on a bottom surface 4 of the light conductive plate 2. The light reflection pattern 7 is composed of grooves 8 each substantially rectangular in section and flat portions 9 each parent between adjacent grooves 8, and is adapted to reflect and emit light rays from the lamp 11 substantially vertically from the top surface 6 of the light conductive plate 2. In the light reflection pattern 7, depths of the grooves 8 differ depending on their position so that luminance is almost uniform at any positions of the light conductive plate 2 independent of a distance from the lamp 11. Specifically, the depth of the grooves 8 is set to increase gradually with an increase in distance from an end surface 3 facing the lamp 11 such that a groove 8 thereof closest to an end surface 5 opposite to the end surface 3 has a maximum depth. In this connections since the grooves 8 of the light reflection pattern 7 formed on the light conductive plate 2 are very fine, they cannot be recognized with the naked eye when a screen is observed. Alternatively, the distance between two adjacent grooves 8 or the width of the flat portions 9 may be set to gradually decrease as the distance from the lamp 11 increases, while the depth of the grooves 8 is set to be constant regardless of the position.
How the light rays travel at the light reflection pattern 7 will now be described with reference to FIG. 10. In order to define the cross-section of each of the grooves 8, it is assumed that a portion corresponding to the bottom surface 4 of the light conductive plate 2 is a virtual plane S, an inclination angle of an inclined surface 30 closer to the lamp 11 (the right side of FIG. 10) is xcex1, and that an inclination angle of an inclined surface 31 closer to the end surface 5 (the left side of FIG. 10) is xcex2.
The inclined surface 30 is set such that a light ray 32 traveling from the right side of FIG. 10 is totally reflected at the inclined surface 30 and travels substantially vertically toward the top surface 6 of the light conductive plate 2 so as to be oriented substantially vertical to the screen. It is proved experimentally that in order to make the light ray 32 travel as above described, the inclination angle xcex1 of the inclined surface 30 may be set to range approximately 40xc2x0 to 55xc2x0. In this case, since the optimum inclination angle a varies depending on the size of the light conductive plate 2, it should be selected appropriately in accordance with the size of the light conductive plate 2.
The inclination angel xcex2 of the inclined surface 31 is set in consideration of a light ray 33 incident on the inclined surface 30 at an angle smaller than a critical angle. Most of the light ray 33 incident on the inclined surface 30 at an angle smaller than a critical angle are not reflected thereat but pass therethrough to exit out from the light conductive plate 2. Here, the inclination angle xcex2 is set such that the light ray 33 having exited out from the light conductive plate 2 reenters the light conductive plate 2 to be incident on an inclined surface 30 of a next groove 8. It is proved experimentally that in order to make the light ray 33 as described above, the inclination angle xcex2 is set to range approximately 60xc2x0 to 90xc2x0.
Here, since the inclination angle xcex1 of the inclined surface 30 is set such that a light ray traveling like the light ray 32 is totally reflected at the inclined surface 30 to be directed toward the screen substantially vertical to the screen as described above, the light ray 33, which reenters the light conductive plate 2 through the inclined surface 31 and reaches the inclined surface 30 is totally reflected thereat to travel in a direction perpendicular to the screen.
When the inclination angles xcex1 and xcex2 are set as described above, light rays going toward the light reflection pattern 7 travels similar to the light ray 32 or 33, and finally exit out from the top surface 6 of the light conductive plate 2 substantially perpendicular to the screen. Also, since there are very few light rays exiting out from the bottom surface 4 of the light conductive plate 2, there is no need to provide a reflection plate, which is an essential component in the conventional spread illuminating apparatus and disposed on the bottom surface 4 of the light conductive plate 2.
The light rays are totally reflected mainly at the inclined surfaces 30 of the grooves 8 to exit out from the top surface 6 of the light conductive plate 2. Since areas of the inclined surfaces 30 and 31 are set to increase gradually as the distance from the lamp 11 increases, an amount of the light rays incident on the inclined surfaces 30 also increases gradually. Accordingly, the light rays can be spread in a uniform manner on the screen in spite of the intensity of the light rays attenuating with an increasing distance from the lamp 11.
A front illuminating means is structured with the light conductive plate 2 configured upside down and with a reflection type liquid crystal display element, instead of a transmission type, being placed under the light conductive plate 2. The mechanism for achieving a uniform spread light emission on the screen is same as that of the back illuminating means described with reference to FIG. 10.
In the spread illuminating apparatus of side light type mentioned above, the luminance of the screen varies largely depending on the viewing angle to the screen at an increased distance from the lamp 11, thereby narrowing its viewing field angle.
It is an object of the present invention to provide a spread illuminating apparatus of side light type, in which the direction of light rays exiting out from the light conductive plate 2 is controlled so as to obtain a minimum viewing field angle for n increased viewing field angle irrespective of the distance from the lamp 11 thereby further improving the image quality of an LCD illuminated by the spread illuminating apparatus.
In order to achieve the above object, a spread illuminating apparatus of side light type according to the present invention may comprise: a light conductive plate made of a light-transmissible material; a bar-like lamp disposed close to and along at least one end surface of the light conductive plate; and a light reflection pattern including a large number of grooves each having an inclined surface and formed on one major surface of the light conductive plate, the inclined surface of some of the large number of grooves including a non-flat portion whose inclination angle varies gradually. According to the present invention, light traveling in the light conductive plate is reflected at the non-flat portion with an appropriate angle suitable for increasing the viewing field angle and exits out from the other major surface of the light conductive plate. Accordingly, the viewing field angle can be increased at the non-flat portion.
Further, in the spread illuminating apparatus according to the invention, preferably, the light reflection pattern may comprise a large number of grooves each shaped substantially triangular in section and a large number of flat portions each present between two adjacent grooves, and may be oriented along the length of the lamp, the depth of the grooves may increase with an increase in the distance from the lamp, and the inclined surface of grooves having a predetermined depth and larger may include the non-flat portion. Since the depth of the grooves increases with the increase in the distance from the lamp, the non-flat portion is provided on the grooves positioned at a predetermined distance and farther from the lamp. Accordingly, the viewing field angle can be increased at the area positioned at a predetermined distance and farther from the lamp.
Further, in the spread illuminating apparatus according to the invention, preferably, the light reflection pattern may comprise a large number of grooves each shaped substantially triangular in section and forming a stair-like configuration as a whole, and the inclined surface of grooves may include the non-flat portion. Accordingly, the viewing field angle can be increased at the area positioned at a predetermined distance and farther from the lamp.
Further, in the spread illuminating apparatus according to the invention, preferably, the non-flat portion may be either convexly or concavely curved. This allows am appropriate selection between the two curve configurations depending on various conditions such as the viewing field angle required, whole configuration of the light conductive plate, and the like.
Further, in the spread illuminating apparatus according to the invention, preferably, the non-flat portion may be arc-shaped in section. With such a configuration, the shape of a cutting tool for forming the grooves can be simplified.
Further, in the spread illuminating apparatus according to the invention, preferably, the non-flat portion may be polygon-shaped in section. With such a configuration, the shape of the cutting tool for forming the grooves can be simplified.
And, in the spread illuminating apparatus according to the invention, preferably, the inclined surface may be constituted either partly or entirely by the non-flat portion. This allows an appropriate selection in consideration of various conditions.